Epitaxial metal oxide buffer layers on substrates with crystalline, polycrystalline, or biaxially-textured metal surfaces are potentially useful where an electronically active layer is deposited on the buffer layer. The term “epitaxial” is used herein and understood by those skilled in the art to mean the growth (method) and placement (apparatus) of a crystalline substance on a crystalline substrate, where the crystalline substance formed follows the crystallographic orientation of the crystalline substrate. Epitaxial crystal growth advantageously permits the formation of crystallographic layers having a high level of crystallographic correlation with respect to an underlying crystalline substrate layer, permitting the formation of improved devices.
The electronically active layer may be a superconductor, a semiconductor, a ferro-electric or an opto-electric material. For example, a biaxially-textured superconductor article to be used for power transmission lines has a multi-layer composition 10, as in FIG. 1. Such deposited superconductor articles most commonly consist of a biaxially-textured metal surface 12, a plurality of buffer layers 14, 16, and a superconducting layer 18. The biaxially-textured metal surface 12, most commonly formed from Cu, Ag, Ni, or Ni alloys, provides support for the superconductor article, and can be fabricated over long lengths and large areas.
Epitaxial metal oxide buffer layers 14, 16 comprise the next layers in the superconductor article. The buffer layers 14, 16 are commonly formed from Y2O3 or CeO2, and serve as chemical barriers between the metal surface 12 and the last layer, the last layer being superconducting layer 18.
Current materials research aimed at fabricating improved high-temperature superconductor articles is largely focused on epitaxial growth of high-temperature superconducting layers on biaxially-textured metal surfaces. A biaxially-textured article can be defined as a polycrystalline material in which the crystallographic in-plane and out-of-plane grain-to-grain misorientations are small (typically less than 20 degrees) but finite (typically greater than 2 degrees). Superconducting articles with current densities (Jc) in excess of 0.1 MA/cm2 at 77 K have been achieved for epitaxial YBa2Cu3O7 films on biaxially-textured Ni or Ni-based alloy surfaces with the use of certain epitaxial buffer layer constructs between the metal surface and the superconducting layer. In previous work, the synthesis of high-temperature superconductor layers capable of carrying a high (at least 0.1 MA/cm2 at 77 K) Jc has required the use of complex, multilayered buffer architectures.
In order to realize a high-temperature superconducting layer, such as YBa2Cu3O7, possessing a Jc greater than approximately 0.1 MA/cm2 at 77 K on a biaxially-textured metal substrate, the buffer layer architecture should be epitaxial relative to the metal substrate and crack-free. Most preferably, the grains of the buffer layer should be crystallographically aligned perpendicular to the plane of the metal substrate (c-axis oriented) and parallel to the plane of the metal substrate (a-b alignment).
Formation of superconductor articles with this orientation begins with the selection of the metal surface 12. The crystallographic orientation of the metal surface 12 is preferably maintained in the buffer layers 14, 16 and the superconducting layer 18, to the maximum extent possible. Numerous conventional processes are currently being used to grow buffer layers 14, 16 on a metal substrate 12. These processes include vacuum methods, such as pulsed laser deposition, physical vapor deposition electron beam evaporation and sputtering. Also, non-vacuum deposition processes, such as chemical solution deposition and chemical vapor deposition can be used for this purpose.
In addition to being epitaxial relative to the biaxially-textured metal surface, buffer layers 14, 16 are preferably chemically compatible with both the metal surface and superconductor, and mechanically robust so as to prevent microscopic crack formation in the high-temperature superconducting layer and the buffer layers. Prior to the present invention, buffer layers that met these objectives have required multilayer combinations of various oxides.
For example, CeO2 has been used to nucleate an epitaxial (001) oriented oxide layer on a biaxially textured (100) Ni surface. However, CeO2 films of over 100 nm thickness form cracks on {100}<001 > textured Ni substrates due to significant differences in the thermal expansion coefficients of the oxide film and the Ni substrate. Cracking has prevented utilization of CeO2 as a single buffer layer.
Also, YBCO films grown directly on a YSZ buffer layer on Ni substrates result in two in-plane orientations. This is due to the lattice mismatch between YBCO and YSZ layers. This generally prevents use of YSZ as a single buffer layer.
An additional buffer layer, such as an epitaxial yttria-stabilized zirconia (YSZ) buffer layer on a CeO2 buffer layer has been used to achieve substantially crack-free superconductor articles. The architecture of YBCO/CeO2/YSZ/CeO2/Ni has been the standard architecture for the rolling-assisted biaxially textured substrate (RABiTS) based YBCO coated conductors. In this arrangement, the superior mechanical properties of the YSZ layer substantially circumvent the microcracking problem, and enable the formation of superconducting layers with a high Jc. The CeO2 layer serves primarily to nucleate a (001) oriented epitaxial oxide on the metal surface.
An alternative multi-layer buffer layer uses conducting SRO (SrRuO3 or Sr2RuO4) and LNO (LaNiO3) buffer layers to form YBCO/SRO/LNO/Ni. The suppression of superconducting critical temperatures (Tc) of 75-80 K for YBCO films grown directly on LNO buffers has prevented the use of LNO as the single buffer layer. Also, the preparation of both SRO and LNO target materials are extremely difficult.
Though effective in forming a high Jc superconductor article, the use of a multilayer buffer architecture, as opposed to a single layer buffer architecture, increases the complexity of the superconductor article fabrication process. Using multiple buffer layers typically requires the use of additional raw materials, as compared to a single buffer layer architecture. In addition, having CeO2 as the nucleating layer tends to permit the formation of microscopic cracks that can limit the maximum Jc of the superconductor article or result in reliability problems during field use.
Epitaxial metal oxides on crystalline or polycrystalline metal surfaces have potential application in fields other than superconductors. Epitaxial metal oxides on crystalline metal surfaces may prove useful where thin epitaxial layers are needed in electronic applications. Furthermore, epitaxial oxide layers on polycrystalline metal surfaces have potential use in tribological or fuel cell applications where the properties of the metal/oxide interface largely determine material performance. For epitaxy on randomly-oriented polycrystalline metal surfaces, the epitaxial relationship involves a grain-by-grain registry of film and substrate crystallographic orientations.